Furfural derivatives of chemical formula (1), including 5-hydroxymethylfurfural (HMF), 5-alkoxymethylfurfural (AlkMF) and 5-acyloxymethylfurfural (AcMF) are interesting chemicals. The furfural derivatives find application as precursor for e.g. furan dicarboxylic acid, an important monomer for polyesters, polyamides and polyurethanes. Alternatively, they can be used as fuel components. HMF has further antibacterial and anticorrosive properties. HMF, AlkMF and AcMF can be derived from sustainable sources. The furfural derivatives may be derived from a variety of carbohydrates, in particular from hexoses, such as fructose and glucose. Raw materials such as starch, cellulose, sucrose or inulin can be used as starting products for the manufacture of hexoses.
Since HMF, AlkMF and AcMF can be obtained from sustainable sources the interest in their production is growing. A process for their production is described in U.S. Pat. No. 7,317,116. This US patent specification describes a process for the preparation of HMF wherein a fructose source, such as high fructose corn syrup, and an organic solvent are heated in the presence of an acid catalyst to achieve the acid-catalyzed dehydration reaction of fructose. The resulting product may then be neutralized to a pH of 7 to 9, e.g. by the gradual addition of sodium hydroxide. In examples the neutralization is carried out to pH values of at least 7.5. Subsequently, the thus neutralized product was subjected to distillation to remove the solvent.
In a different embodiment U.S. Pat. No. 7,317,116 describes the preparation of an R′-oxymethyl furfural ether wherein R′ may represent alkyl, by combining a fructose source and an R′—OH solvent and by contacting the combination thus obtained with a solid acid catalyst bed in a chromatographic column. By heating the admixture in the chromatographic column fructose is dehydrated to form R′-oxymethylfurfural ether.
In U.S. Pat. No. 8,877,950 a process is described wherein ethers of 5-hydroxymethylfurfural are manufactured by reacting a fructose-containing starting material with methanol, in the presence of a catalytic or sub-stoichiometric amount of a homogenous acid catalyst, wherein water is present as solvent in addition to the alcohol, wherein the ratio of alcohol/water-solvent is from 50:1 to 10:1, wherein the method is performed in a continuous flow process at a temperature of 175 to 225° C. and at a residence time in the flow process from 1 minute to 10 minutes. Very suitably the homogenous acid catalyst is sulfuric acid.
Further, furfural derivatives of the chemical formula (1) wherein R represents an acyl group, have been described in U.S. Pat. No. 8,242,293. Such derivatives can be prepared by reacting a fructose and/or glucose-containing starting material with a carboxylic acid in the presence of an acid catalyst in a continuous mode, wherein water is present in a small proportion. The carboxylic acid may be selected from e.g. C1-C6 carboxylic acids. The reaction yields esters of HMF, wherein the acyl moiety of the carboxylic acid is bonded to the oxygen atom of the oxymethyl group at the 5-position, herein referred to as AcMF. As shown in an example, sulfuric acid may be used as the acid catalyst.
The product obtained in any of these processes includes by-products, in addition to HMF and/or AlkMF and/or AcMF. A competing side reaction is the polymerization of HMF, AlkMF or AcMF and/or the hexose to form humin polymers. Humin polymers or humins are the colored bodies which are believed to be polymers containing moieties from hydroxymethylfurfural, furfural, carbohydrate and levulinic acid. Humins are obtained as insoluble solid material. As shown in the examples of U.S. Pat. No. 7,317,116, other by-products may include levulinic acid, levulinate esters and formic acid. Such by-products add to the acidity of the reaction mixture, which may already be acidic in view of the presence of the acid catalyst. It was found that under such acidic conditions degradation reactions occur which affect the yield of the desired products HMF, AlkMF and/or AcMF. Therefore, it has been proposed to neutralize the reaction mixture.
U.S. Pat. No. 7,317,116 teaches in particular a process for the preparation of HMF by i) combining a fructose source, an organic solvent, and an acid catalyst to provide a reaction mixture; ii) heating said reaction mixture to a temperature and for a time sufficient to promote a dehydration reaction of fructose in said fructose source to form a first product mixture; iii) neutralizing the pH of the first product mixture to a pH of about 7 to 9; iv) distilling the first product mixture after neutralizing the pH to remove said organic solvent remaining in the first product mixture; and v) purifying said product mixture to provide a second product mixture comprising greater than 60% by weight of HMF. In one embodiment, the product is adjusted to a neutral pH after removing the ion-exchange resin from said product mixture, and before being subjected to a distillation to remove the organic solvent.
According to U.S. Pat. No. 7,317,116 neutralization is desirable as it allows for product recovery by distillation without heat-catalyzed degradation or polymerization leading to tarry degradation products and resinous solids, i.e. humins. The neutralization step also stated to allow for product recovery with a flowing agent without such degradation or polymerization.
However, it has now been found that neutralization to alkaline pH values may still lead to degradation reactions of the desired furfural derivatives. When setting out to avoid degradation the inventors of the present invention surprisingly found that degradation reactions do not occur when the neutralization is done to a certain extent only.